In the abovementioned technical field, a tool fixation assisting tool having a large-area contact surface which comes in contact with an inclined surface of a ring-like tool is disclosed in each of patent literatures 1 and 2. The purpose of this assisting tool is to provide an inner surface bead cutting tool which enables the use of even an ordinary flat bolt regardless of the size of a ring-like tool, and can strongly fix the ring-like tool with a large contact area.